Conventionally, as an ink for writing instrument for forming an image which changes color, those utilizing a photochromic material (e.g., Patent Document 1) are known.
A writing image formed with such an ink utilizing a photochromic material is allowed to develop color by irradiation with sunlight and maintains the coloring state under the irradiation with sunlight. However, when it is left at a place which is not irradiated with sunlight, the coloring state is not maintained, and it is spontaneously decolored reversibly to return the original handwriting, and under normal conditions, the coloring state is not maintained, therefore, the handwriting at writing could not be confirmed visually.
[Patent Document 1]
JP-A-8-156479